Power ultrasonic transducers are applied against environmental conditions or loads in order to utilize the ultrasonic energy to accomplish the desired function.

One way to characterize the interaction of the transducer with the load/environment is to use the transducer capacitance.

When a transducer is loaded, it will more constrained - this will reduce the capacitance and this reduction can be used to characterize the extent of loading. The load won't be significant enough in all cases to cause a measurable change in capacitance, but you might be surprised as to how useful this measurement can be in your application. Try it out!